What I read from bits and pieces of information released by Igiss before 4.85's release was that there was a small passage through the mines that they made to allow Junkers to secretly pass through the fields to bring in anything that they managed to steal.
Q: Is Gallia as technologically advanced as the rest of Sirius?
A: Yes. Moreover, in many ways Gallia is more advanced than the other Houses. To achieve technological superiority, Gallia used spies to collect data on various technologies that were used in Sirius. Technology of jump gates and trade lanes, for example, was successfully implemented to build Gallia's own space transit system.
Gallia was not set back by delayed colonization. In fact, larger sleeper ship allowed to transfer about twice more people than other nations could transfer, and right now the population of Gallia is the largest of all Sirius houses.
Q: How was Gallia isolated from the rest of Sirius? Why was it never discovered?
A: Gallia spent decades to construct four huge mine fields to hide both paths, and disguise them to look like ancient mine fields present in other areas of Sirius (for example, in Newcastle). Radars were unable to detect jump holes that lead into Gallic systems. This is why Sirius explorers were never able to find paths between known Sirius systems and Gallia.
Q: What was the purpose of isolation?
A: Purpose was to achieve technological and military superiority over the rest of Sirius Houses. Using information collected by their spies, along with their own research, Gallic engineers and scientists have indeed surpassed the current level of Sirius technology in many areas. However, this also meant that wellbeing of simple citizens was far from being number one on the government's priority list.
Q: The Gallic agents who come to and from Sirius should somehow pass through the mine field. How do they do it if no one else is able to?
A: They use special ships and narrow pathways between the mines that only they know of. Ships and paths are held in secrecy, and players won't have access to them (at least not in the next version). Many of the paths were destroyed anyway during the events that indicated breach of the 7-hundred-year isolation.
Q: It appears that Gallia had an extensive espionage network in Sirius. Do we know anything about the spies?
A: Espionage network was built by a well-known faction currently present in many areas of Sirius. This faction was initially founded in Gallia by a group of colonists who had anglo-saxon origins. Later, Gallic intelligence took the faction under close control. Presence of this faction was extended to Sirius, namely Bretonia and Liberty; nowadays, most members of this faction are completely unaware of its true origins and goals.
That section is more or less everything to deal with Junkers involvement with Gallia and their passage between Gallia and Sirius.